1. Field of the Invention
The present invention relates to a polymerizable composition that can be polymerized and cured by light or heat. More specifically, it relates to a photo- or thermo-polymerizable composition that can be used as a recording layer of a “Computer-to-Plate” planographic printing plate precursor with high sensitivity. In “Computer-to-Plate” system, images are recorded on a planographic printing plate by image-wise irradiating with lasers based on digital signals from computers or the like.
2. Description of the Related Art
Solid lasers, semiconductor lasers and gas lasers that emit UV light, visible light, and infrared light in a wavelength range of 300 to 1200 nm are widely available and they are high in power and small in size. These lasers are useful as a recording light source when direct plate making is performed from digital data of a computer or the like. There are various studies on image recording materials that can respond to these various laser lights; as typical image recording materials, acid catalyst crosslinking negative image recording materials (for instance, JP-A No. 8-276558) that allow recording with an infrared laser of a photosensitive wavelength of 760 nm or more are known. Furthermore, examples of recording materials sensitive to UV light or visible light in a range of 300 to 700 nm also include a lot of radical polymerizable negative image recording materials (for instance, U.S. Pat. No. 2,850,445 and JP-B No. 44-20189).
As high sensitive radical polymerizable recording materials particularly suitable for a printing plate, usually, polymerizable compositions that contain an alkali-soluble polymerizable polymer binder and a polymerizable crosslinking agent (monomer, oligomer) are used. In such radical polymerizable compositions, while an increase in an amount of the polymer serving as a binder is generally effective in improving storage stability, on the other hand, a tendency toward lower sensitivity is observed. On the contrary, when a content of the polymerizable crosslinking agent is increased to attain higher sensitivity, storage stability is likely to deteriorate. Accordingly, the present situation is that a balance between storage stability and high sensitivity is attained by controlling a ratio of the binder to the polymerizable crosslinking agent.
On the other hand, as highly sensitive radical polymerizable photo- or heat-sensitive materials suitable for a recording layer of the printing plate, usually, combinations of alkali-soluble polymerizable polymer binders and polymerizable crosslinking agents (monomer, oligomer) are used. With the intention of attaining high sensitivity, for instance, JP-A No. 2000-187322 proposes a technique in which one that is endowed with a radical polymerizable group within a molecule is used as a binder. When the binder is endowed with the radical polymerizable group, since the binder that is a polymer participates directly in the crosslinking, an augmentation of the crosslinking reaction due to a molecular weight effect is caused, and thereby a little bit higher sensitivity can be assuredly obtained; on the contrary, since the photosensitive materials become thermally unstable, there are concerns in that storage stability may deteriorate owing to the higher gelling efficiency of the crosslinking binder, and deterioration of the developability of non-image areas interposed between image fine lines due to a crosslink structure formed in the polymer may result in less sharpness of the fine lines, that is, lower image quality. This is a general dilemma when the polymers are used; even with the above technique, the essential reciprocality between the stability and the sensitivity cannot be avoided. This is considered that generally in the case of the polymers, when the polymerizability thereof is improved, storage stability that is observed in non-polymers tends to decrease, as a result, the balance between the sensitivity and storage stability is deteriorated.
Usually, photo-radical polymerizable compositions allow a curing reaction to occur with high sensitivity; however, since polymerization inhibition due to oxygen in the air is likely to occur, it is common to dispose an oxygen-shielding layer on an image formation layer. However, the disposition of the oxygen-shielding layer causes fogging due to dark polymerization, resulting in deterioration in storage stability.
Furthermore, in a recording layer obtained by forming a film from the photo-polymerizable composition, a curing reaction due to light proceeds sufficiently in the neighborhood of a surface thereof. However, in the depths of the recording layer, there is a problem in that since a sufficient curing reaction does not proceed, high press life is difficult to attain.
Thus, in photo-polymerizable compositions that utilize a curing reaction due to light exposure, a technique that can sufficiently accomplish both objects of high sensitivity and storage stability has not been obtained at present.